The present invention generally relates to methods of and compositions for de-icing and anti-icing surfaces, and particularly to methods of and compositions for de-icing and anti-icing which do not rely on glycols as de-icing and anti-icing agents. More specifically, the present invention relates to methods of and compositions for de-icing and anti-icing surfaces using one or more polyhydric alcohols in combination with one or more non-potassium, low carbon percentage organic compounds, and optionally, one or more non-potassium, non-halide inorganic salts and alcohols.
Ice build-up on aircraft wings, runways and roadways is a significant problem. First, ice build-up threatens the safety of travelers by causing or contributing to accidents. Second, ice build-up increases the costs and time delays of travel. These problems are of special concern to the airline industry.
Ice build-up may be removed from surfaces by "de-icing" processes or compositions, or reduced or prevented from forming by "anti-icing" processes or compositions (collectively, "anti-icing"). Anti-icing and, in particular, aircraft anti-icing may be accomplished by (1) mechanical, (2) electrical, or (3) chemical means.
Mechanical methods physically remove ice by directing heat to the surface (e.g., hot-air impingement) or pneumatics (e.g., alternatively inflating and deflating air-filled bags on wings/tail surfaces). Anti-icing technologies include the use of hydrophobic/icephobic substances like chitin/chitosan paints (for airplanes) or silicone polymers (for runways). Electrical methods include heating or electromagnetic repulsion. While attractive for not requiring the application of chemicals (with the attendant environmental and health concerns), electrical and mechanical methods may not be effective under conditions of excessive icing or snowfall. In addition, these methods may require the availability of a large power supply (which is not always practical) or a large-volume airport/airbase.
Chemical methods traditionally have included the application of compositions of solid salts or liquid solutions which melt or inhibit the formation of ice. Known chemical anti-icing products rely on compounds like glycols (e.g., ethylene, propylene, diethylene, alkylene), urea, calcium magnesium acetate (CMA), sodium formate, and potassium acetate. While chemical methods overcome many of the difficulties inherent in mechanical or electrical anti-icing methods, their use has certain other disadvantages. Notably, chemical compositions may create safety and health problems, may be of limited effectiveness, may cause environmental harm, may physically or chemically damage the runway/vehicle, and/or may not be cost-effective.
In recent years, public concern and attention to groundwater and waterways pollution has increased. One source of such pollution is the anti-icing compositions used to prevent or remove ice build-up from aircraft and runways.
Chemical anti-icers of the type used until now may contribute to environmental degradation. Glycols, in particular, are environmentally problematical and their use as aircraft anti-icers is scrutinized under the Clean Water Act. Some like ethylene glycol, long used by the Air Force as an anti-icer, are toxic. Others, like propylene glycol, create environmental problems.
The degree to which a chemical creates Biochemical Oxygen Demand ("BOD") in the environment denotes that chemical's environmental toxicity. Specifically, BOD is a measure of "the amount of dissolved oxygen required to meet the metabolic needs of aerobic microorganisms in water rich in organic matter, such as sewage." McGraw-Hill Dictionary of Scientific and Technical Terms, p.215 (Sybil Parker, ed., 4th ed. 1989). That is, a composition that creates a high BOD fosters the growth of (potentially harmful) microorganisms in the environment while depleting available oxygen. Likewise, a composition which creates a high BOD rate fosters a faster rate of growth of (potentially harmful) microorganisms and oxygen depletion. As used herein, compositions are described as "having" a high BOD or a high BOD rate.
Ideal anti-icing compositions for use on aircraft or runways possess: (i) high freezing point depression ("FPD"); (ii) low Biochemical Oxygen Demand("BOD"); (iii) low solution conductivity; (iv) a high viscosity; (v) low toxicity; and (vi) low corrosivity.
Complicating matters is the fact that these qualities vary based on numerous interacting chemical and physical properties. It is not predictable as to which combinations will yield the best results. For example, while a large FPD is desired, a high solution conductivity is not. Increasing the non-potassium salt content increases both the FPD and the solution conductivity. Therefore, concentrations of the various components of the present invention had to be optimized to reach the desired properties.
Glycols currently used as anti-icers have a high BOD, can be toxic to aquatic organisms, and can lead to eutrophication of waterways. In addition to high BOD, glycols can cause rapid oxygen depletion in water (oxygen "crashes") after a longer enzymatic assimilation period than glycerol, for example. This longer assimilation period allows the glycols to reach waterways and stagnant bodies of water before crashing the oxygen. Certain glycols used as anti-icers have longer assimilation periods because they are uncommon or not found in nature, and the microbes which decompose them require an enzymatic adaptation period.
In contrast, some chemical anti-icing compositions employ alkali organic salts like potassium acetate. While these compositions have a reasonably low BOD, they have other serious drawbacks. For example, their electrical conductivity facilitates certain electrochemical reactions, which can corrode metals (e.g., steel, copper, aluminum, magnesium) and metal alloys commonly used in aircraft. As a result, these compositions may have practical use as runway or pavement anti-icers, but are unacceptable for direct aircraft anti-icing.
A gelatinous composition using glycols or glycerol has been shown to be useful to prevent ice-build-up on aircraft wings. See, for example, U.S. Pat. No. 2,101,472. In addition, the gelatinous composition described in U.S. Pat. No. 2,373,727 contains glycol and/or glycerol and other organic compounds. Neither patent expressed a preference for glycerol for the purpose of addressing the environmental problem of high BOD. In fact, the organic compounds of the latter patent may exacerbate the BOD problem where the solution is not completely oxygenated. Because the organic compounds lower the flashpoint, safety concerns also are increased. Moreover, neither composition comprehends the use of a fluidized glycerol/water solution having low molecular weight salts.
U.S. Pat. No. 3,362,910 describes an automotive radiator antifreeze composition containing water, glycol or glycerol, corrosion inhibitors--alkali metal silicates, a mercaptan, sodium tetraborate, and an alkali carbonate or hydroxide. No suggestion is found for choosing glycerol or other low-carbon percentage compounds to reduce BOD. Likewise, conductivity and corrosion problems associated with placing solutions containing alkali metals in contact with metals are not addressed. Importantly, this composition does not have the higher viscosity required (thereby yielding better adhesion) for anti-icing applications.
U.S. Pat. Nos. 4,117,214; 4,163,079 and 4,439,337 teach ice-inhibiting compositions containing (i) polyhydroxy or monoalkyl ethers and (ii) water soluble, non-volatile organic compound(s). The composition of U.S. Pat. No. 4,117,214 reduces the strength of ice. Dilute aqueous solutions (0.3%-5.0%) which freeze weakly are used. The composition does not melt ice or prevent ice formation, nor does it provide for a freezing point depression of water of 20.degree. C. or greater.
The composition of U.S. Pat. No. 4,163,079 reduces the slippage of particulates on conveyor belts, the belts requiring periodic retreatment. Specifically, the composition is added in dilute concentration to reduce the compression strength of ice. The water soluble, non-volatile organic compound contains a hydrophilic group like an amine, carboxyl, or carboxylate group or polydimethyl siloxane. This composition contributes to the environmental problems previously-mentioned. The amines and other nitrogen groups contribute to eutrophication of waterways, and glycol causes BOD problems.
The composition of the U.S. Pat. No. 4,439,337 prevents particulate matters and surfaces from freezing together by reduction of ice compression strength. The composition contains polyhydroxy compounds, a single non-volatile organic compound with a hydrophilic group, an optional inorganic halide salt to depress the freezing point of water and an organic polymer thickener to adjust the viscosity. This composition also contributes to the same environmental problems discussed above. Ethylene glycol is the preferred polyhydroxy compound, notwithstanding its toxicity. Minimizing BOD is not addressed. Furthermore, the salts employed to lower the freezing point are corrosive to metals and, consequently, unsuitable for treating most aircraft surfaces or for use in other heat transfer applications potentially used for sub-zero temperatures such as an automotive radiator.
The glycerol foam of WO 87/04450 is provided as an anti-icing agent. This composition employs alkali salts of fatty acids (long-chain, high molecular weight organic compounds) and amines/amides. Again, this composition ignores the environmental problems resulting from the use of amines/amides and from not minimizing the BOD.
The anti-icing composition of WO 96/23043 contains: (i) water, (ii) a polyhydric alcohol, (iii) an organic thickener, and (iv) other additives. This composition is addressed to the field of "aqueous, non-electrolytic solutions" and not to dissociating, water-soluble salts. While glycerol is a polyhydric alcohol, this composition does not comprehend or address the rheological (i.e., viscosity) and environmental advantages of glycerol over glycol. This composition is burdened with the same environmental and effectiveness problems discussed above.
To summarize, the disadvantages of the background art are that currently available compositions for preventing ice-build-up: (i) damage the environment; (ii) are not effective at lower temperatures; (iii) are not sufficiently versatile; (iv) cause health or safety problems; (iv) damage the treated surfaces; and/or are (v) not cost-effective.
In short, there are a number of problems with the currently available anti-icing compositions. Damage to the environment is caused by (i) chemical components which, like glycol, have high BOD or high BOD rates and/or damage soil and waterways, and (ii) nitrogen-bearing compounds like amines or amides which lead to eutrophication of waterways. Some compositions simply are not effective at lower temperatures (e.g., cannot depress the freezing point 20.degree. C. or greater). Other compositions merely weaken ice formation or compression and are less useful for preventing or removing ice build-up. Still other compositions do not have the proper viscosity necessary for aircraft and/or roadway application. Certain compositions described are toxic, while others are dangerous because of lower flashpoints. Some are corrosive to metals and/or other aircraft materials or have high solution conductivity, limiting their utility. Of course, cost concerns minimize the utility of certain compositions.
It is, therefore, a primary object of the present invention to overcome these disadvantages by providing an anti-icing composition and a method for its use.
A further object is to provide a composition which contains no glycols or polymers thereof. A related object is to provide a composition which minimizes BOD and the rate of BOD. An additional object is to provide a composition which has a freezing point depression of 20.degree. C. or greater.
Another object is to provide a composition which allows a range of viscosities without requiring additional thickener additives, and to provide a composition which has a higher flashpoint than glycol-based compositions.
An important object is to provide a composition which maximizes the freezing point depression while minimizing the corrosivity, solution conductivity, BOD and rate of BOD. A related object is to provide a composition which will not cause eutrophication of waterways. A related object is to provide a composition which provides for more rapid environmental assimilation.
These and other objects of the invention will become apparent in the description of the preferred embodiments below.